Fatherhood can influence health and longevity in men. The mechanisms underlying these effects are unknown; however, as human and other mammalian fathers undergo predictable changes in several metabolically important hormones, one possibility is that these endocrine profiles influence energy homeostasis, metabolism, and body composition, which in turn can affect health and longevity. The proposed studies will characterize the effects of fatherhood in the California mouse (Peromyscus californicus), a monogamous rodent in which fathers invest extensively in their offspring and undergo systematic changes in hormone levels and body mass. Specifically, this research will test the hypotheses that fatherhood alters: plasma levels of metabolically important hormones (Aim 1); energy homeostasis, metabolism, body mass, and body fat (Aim 2); behaviors that may contribute to these energetic and metabolic effects (Aim 3); and that these consequences of fatherhood are modulated by paternal parity/age, offspring age, and stress. Experiment 1 will use a longitudinal design to characterize the effects of fatherhood on physical (body mass, body composition), energetic (resting and maximal metabolic rates, daily energy expenditure), hormonal (corticosterone, testosterone), and behavioral (ingestive behavior, activity levels) measures. Data will be collected at five time points, spanning approximately a 9-month period, from fathers (housed with a mate and pups) and nonbreeding males (housed with an ovariectomized female). Half of the mice in each group will be exposed to a chronic, intermittent noise stressor, and the other half will be maintained under standard laboratory conditions. Experiment 2 will use a cross-sectional approach to further characterize possible hormonal, metabolic, and behavioral mechanisms underlying the anticipated effects of fatherhood on body mass, body composition, and energetics. Specifically, it will characterize 1) circulating levels of the metabolic hormones leptin, adiponectin, triiodothyronine, and thyroxine; 2) blood triglyceride, cholesterol, and glucose levels; 3) the propensity to ingest high-fat and high-sucrose substances; and 4) organ masses and distribution of fat stores, in nonbreeding males, new fathers, and experienced fathers. Depending on the results of Experiment 1, half of the mice in each group may again be exposed to a chronic, intermittent noise stressor. These studies are expected to demonstrate that fatherhood in P. californicus alters metabolic hormone levels, energy homeostasis, metabolism, body composition, and behavior. They will also clarify how these effects are modulated by paternal age/parity, offspring age, and chronic stress. The results will provide unique insights into the biology of mammalian fatherhood and may eventually lead to an improved understanding of the effects of fatherhood on health and longevity in men, as well as health outcomes in their offspring.